We seem to have a lot of embryology going, so might as well keep it up! Last week, we went discussed what happens in week 2, specifically the bilaminar disc and the chorion cavity. This week, let’s keep moving forward and take a look at what happens in the third week of development. If week two had the bilaminar disc, what do you think week three has? That’s right. The trilaminar disc. It’s also important to note that this is usually the week following the first missed menses.

 

The important things that are developed during this process is the three definitive germ layers. Those are the ectoderm, mesoderm and endoderm. So, we’re basically taking the two layers we already had and developing it into a three layers. It’s not exactly like just adding another layer, but its not that far off, either.

 

Step one is forming the primitive streak. That appears on the surface of the epiblast and begins this process which we call gastrulation. The primitive streak looks like a little circular depression (the primitive node) with a line going to the nearest edge of the surface. This also indicates the first orientation of what will become the fetus. The direction the line points will end up being the caudal (butt) end and the untouched area ends up being the cranial (head) end. The way this is formed is by the epiblastic cells proliferating and making their way to the end of the embryonic disc. So, that depression and the valley we discussed actually has a ridge that goes all around it. The depression in the primitive node area is called the primitive pit and the groove that forms the valley of the streak is the primitive groove.

 

Next, those epiblast cells are going to keep going. Moving up and over the ridges and into the depressions, they’ll actually start to go in between the epiblast and the hypoblast, pushing them apart. As they keep filling in, they’ll spread out in all directions, like when you pour brownie batter in a cake pan. When this happens, we are forming the ectoderm (which was the epiblast layer), the mesoderm (the layer in between), and the endoderm (which takes the place of the hypoblast).

 

Inside, then, cells are forming mesenchyme. Mesenchyme are essentially pluripotent cells. Pluripotent means that they can grow up to become anything. Those are going to move in the same fashion as the epiblast cells that did the original infiltration. When they reach the edges of the disc they’ve formed the intraembryonic mesoderm, sometimes just called the embryonic mesoderm.

 

Random, but it might be good to note here that the endoderm (where the hypoblast was) is in the roof of the umbilical vesicle. It’s important to keep track of where everything is!

 

At this same time, there is also a little divot forming at the cranial end of the embryo. That is the prechordal plate and is also called the oropharyngeal membrane. It’s where our mouth will end up being. Mesoblasts (cells of the mesoderm) are going to start heading up in that direction. They’re forming a long flat layer as they go which is the notochrodal process. That process is going to eventually fold over on itself and unite with the cardiogenic area of the cardiogenic mesoderm. We’ll work on this more later, probably. Doing the embryonic development of the heart is a bit of an undertaking.

 

Okay, back to the primitive streak, caudal to that, the endoderm and ectoderm are going to fuze to make the cloacal membrane. This is the future site of the anus. Now, eventually the primitive streak is going to decrease in size and become no big deal. In fact, by the end of week 4, it should degenerate and disappear. Interesting side note, though, sometimes it doesn’t and leaves a remnant all the way until birth. Look up sacrococcygeal teratoma and you’ll see examples of it. They’re the most common form of tumors in newborns and usually happen with females. Most of them are benign, though.

 

Alright, so more stuff to form. Let’s work on starting our brain and spinal cord. For that, we’re going to make the notochord. It runs along the longitudinal axis of the embryo and already, it will start giving it some rigidity. Again, mesenchymal cells are going to start heading toward the cranial from the primitive pit to form the notochordal process. That keeps growing until it reaches the prechordal plate. Our primitive pit that we started from? It is going to extend to the notochordal process until it connects and forms the notochordal canal. Think of it as a tubelike structure now.

 

The floor of the notochordal process is going to fuse with the endoderm underneath and then degenerate. So, now it’s filled with cavities, which are then going to connect and the floor will disappear all together. Now the notochordal process flattens and we’ve got that notochordal plate!

 

Not quite done yet, though. Now those notochordal plate cells are going to start proliferating and fold for the notochord. Once it is complete, the canal disappears and the notochord will detach from the endoderm. The notochord persists in adults as the nucleus pulposus. That is the center of your intervertebral discs (surrounded by the annulus fibrosus).

 

We haven’t actually done everything in week 3, but this might be enough for now. Next time, we’ll keep going and tackle the process of neurulation!